1. Field of the Invention
This invention is directed to electrocatalytic electrodes, particularly cathodes useful in electrolysis cells such as a chlor-alkali cell and methods for preparing these cathodes and a method of activating a substrate prior to electroless deposition of a metal.
2. Description of Related Prior Art
The importance of efficient and durable electrodes for use in chlor-alkali membrane or diaphragm electrolytic cells is readily apparent when it is considered that millions of tons of chlorine and caustic soda are produced every year, mainly by electrolysis of aqueous solutions of sodium chloride.
The most widely used chlor-alkali processes employ either diaphragm or membrane type cells. In a diaphragm cell, an alkali metal halide brine solution is fed into an anolyte compartment where halide ions are oxidized to produce halogen gas. Alkali metal ions migrate into a catholyte compartment through a hydraulically-permeable microporous diaphragm disposed between the anolyte compartment and the catholyte compartment. Hydrogen gas and aqueous alkali metal hydroxide solutions are produced at the cathode. Due to the hydraulically-permeable diaphragm, brine may flow into the catholyte compartment and mix with the alkali metal hydroxide solution. A membrane cell functions similarly to a diaphragm cell, except that the diaphragm is replaced by an hydraulically-impermeable, cation selective membrane which selectively permits passage of hydrated alkali metal ions to the catholyte compartment. A membrane cell produces aqueous alkali metal hydroxide solutions essentially uncontaminated with brine.
Electrolytic cells fail to realize the degree of efficiency which can be theoretically calculated by the use of thermodynamic data. Production at the theoretical voltage is not attainable and a higher voltage, i.e., a so-called overvoltage, must be applied to overcome various inherent resistances within the cell. Reduction in the amount of applied overvoltage leads to a significant savings of energy costs associated with cell operation. A reduction of even as little as 0.05 volts in the applied overvoltage translates to significant energy savings when processing multimillion-ton quantities of brine. As a result, it is desirable to discover methods which will minimize overvoltage requirements.
It is known that the overpotential for an electrode is a function of its chemical characteristics and current density. See, W. J. Moore, Physical Chemistry, pp. 406-408 (Prentice Hall, 3rd ed. 1962). Current density is defined as the current applied per unit of actual surface area on an electrode. Techniques which increase the actual surface area of an electrode, such as acid etching or sandblasting the surface of the electrode, result in a corresponding decrease of the current density for a given amount of applied current. Inasmuch as the overpotential and current density are directly related to each other, a decrease in current density yields a corresponding decrease in the overpotential. The chemical characteristics of materials used to fabricate the electrode also impact overpotential. For example, electrodes incorporating an electrocatalyst accelerate kinetics for electrochemical reactions occurring at the surface of the electrode.
Various methods designed to reduce the overvoltage requirements of an electrolytic cell have been proposed including decreasing the overpotential requirements of the electrodes relating to their surface characteristics. In addition to the physical characteristics of the surface of the electrode, the chemical characteristics of the surface of the electrode can be selected to reduce the overpotential of the electrode. For instance, roughening the surface of the electrode decreases overpotential requirements. The platinum group metals are particularly useful to reduce overpotential requirements when present as the metal, alloys, oxides or as mixtures thereof on the surface of an electrode.
Electrodes are usually prepared by providing an electrocatalytic coating on a conducting substrate. The platinum group metals, such as ruthenium, rhodium, osmium, iridium, palladium, and platinum are useful electrocatalyst. For example, in U.S. Pat. No. 4,668,370 and U.S. Pat. No. 4,798,662 there are disclosed electrodes useful as cathodes in an electrolytic cell. These are prepared by coating an electrically conducting substrate such as nickel with a catalytic coating comprising one or more platinum group metals from a solution comprising a platinum group metal salt. Both of these patents disclose electrodes designed to reduce the operating voltage of an electrolytic cell by reducing the overpotential requirements of the electrode. U.S. Pat. No. 4,668,370 also discloses means to overcome the poor adhesion of platinum group metal oxides to non-valve metals when the platinum group metal oxides are coated by electrodeposition from a plating bath. In addition, U.S. Pat. No. 5,035,789, U.S. Pat. No. 5,227,030, and U.S. Pat. No. 5,066,380 disclose cathode coatings which exhibit low hydrogen overvoltage potentials. Metallic surfaced substrates utilized as electrode bases can be selected from nickel, iron, steel, etc. These non-valve metal substrates are disclosed as effectively coated utilizing a non-electrolytic reduction deposition method in which a water soluble platinum group metal salt alone or in combination with a platinum group metal oxide in particulate form is deposited from an aqueous coating solution having a pH of less than about 2.8.
A desirable characteristic of a cathode coating is high porosity with large internal surface areas. Large internal surface areas result in lower effective current density and, accordingly, lower overpotentials. Another result of a porous electrode is higher resistance to impurity poisoning. Rough outer surfaces of a typical porous electrode render difficult the electrodeposition of metal ions as impurities and the large internal electroactive surface areas are not easily accessible to the impurity ions present in the electrolyte because of long pathways for diffusion.
Raney nickel is an example of a porous electrode. In use, Raney nickel porous cathode coatings consisting of non-noble metals such as Raney nickel or Raney cobalt show reduced performance characteristics after shut down of an electrolytic cell. The reduced performance is apparently caused by the oxidation of the nickel or cobalt to the hydroxide during the electrolytic cell shut down period.
Zero-gap electrolytic cells have recently found acceptance industrially. In these cells, both the anode and the cathode are placed in contact with the cell membrane. This configuration avoids the overvoltage problems associated with electrolyte resistance in the older gap cells in which there is a space between the electrode and the membrane. Cathode coatings on thin substrates allow very close contact between an electrode and a membrane without damage to the cell membrane. Because of the thin electrode substrate and because of the requirement that the coating remain adhered to the electrode substrate while exposed to a cell membrane over a large membrane surface, the adhesion of the coating to the electrode substrate must be very tenacious to avoid loss of coating during operation of the electrolytic cell.
It has been found that a durable, porous electrode can be effectively prepared by utilizing a two step method in which two coating layers are applied, each coating layer interpenetrating the adjacent coating layer.
Also disclosed herein is a method of applying an electroless metal coating solution to plate a metal on a non-conductive substrate.
As disclosed in U.S. Pat. No. 4,061,802 and U.S. Pat. No. 4,764,401 palladium chloride has been used to activate plastic or metal substrates prior to nickel plating by electroless deposition. Jackson discloses a water soluble palladium sulfate/polyacrylic acid catalyst system for copper plating of printed circuit boards in J. Electrochemical Society 137, 95 (1990).
In U.S. Pat. No. 4,764,401, organometallic palladium compounds are disclosed as useful to activate a plastic substrate prior to electroless plating of a metal thereon. The palladium compounds are applied to the plastic surface to activate the surface so that an improved rate of electroless plating can take place. The prior art use of organometallic compounds of palladium has the disadvantage that such small molecules tend to be absorbed unevenly on the plastic surface. In addition, subsequent to application of the organometallic compounds of palladium from a solvent solution, crystallization of the molecules can occur. This can cause segregation of the catalyst and leave areas of the plastic surface uncovered by the organometallic palladium compound activator. Such segregation of the palladium activator can also cause growth in the size of the activator molecules and loss in coverage on the plastic surface area. The use of an amorphous polymer instead of the organometallic compounds of palladium overcome these problems simply because an amorphous polymer forms a relatively uniform film on the plastic substrate. Ligands on the amorphous polymer chain can be used to bind the palladium compound and distribute them evenly over the surface of the plastic substrate.
The use of water soluble amorphous polymers, such as polyacrylic acid, as disclosed by Jackson, cited above, in order to incorporate a palladium compound as an activator compound on a plastic substrate also results in difficulty. Such polymer coatings tend to release from the plastic surface carrying the palladium compound activator with it. When this occurs, a plating reaction in the plating solution is initiated. This is undesirable as it results in loss of activity of the bulk solution and can cause inferior coatings on the plastic substrate.
Accordingly, a water insoluble polymer rather than a water soluble polymer is superior as a carrier for the activating palladium compound prior to plating on a plastic surface.